Sometimes, you want to know exactly who's coming. In other cases it's enough to know how many people are coming. If that's the case, then RSVPing might not be the best option. The main reason is that the conversion rate of RSVP events is lower than without. And the benefit is you can still measure how many people added the event to their calendar.
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So - if you're promoting an event and you don't need to know exactly who's coming - use the standard add-to-calendar functionality without RSVP.